Create your own C Header file

See the following simple example to learn: (tested in gcc)
File 1: myLib.c

int sum(int a, int b)
{
      return a+b;
}

File 2: myLib.h

int sum(int, int);

File 3: testLib.c

#include<stdio.h>
#include"myLib.h"
int main()
{
      printf("%d\n",sum(10,20));
      return 0;
}

Commands to run:

gcc myLib.h myLib.c testLib.c
./a.out